The present invention relates generally to devices and systems for controlling and regulating the conversion of power from an AC source to a load, typically a DC motor. More particularly, the present invention relates to devices which control the conduction of controllable rectifier, e.g., thyristor, bridges placed between the source and the motor and, in particular, methods for compensating for inaccuracies caused by voltage notches during bridge commutation.
Motor control systems of the type described above typically include at least one rectifier bridge connecting the motor windings to alternating voltage supply lines. For a conventional three phase motor, each AC phase line is generally coupled to the motor by a pair of connected thyristors. That is, in a three phase system, six thyristors are required to transfer power from the source to the load, each for one half of each phase. A thyristor, such as a silicon controlled rectifier (SCR), is generally defined as a switchable diode controlled by a gate element. Each thyristor presents a relatively high blocking impedance to the flow of electrical energy until it is forward biased by a trigger current being applied to its gate element. A digital control circuit typically determines the proper time to trigger the thryistors during each half-cycle of the supply line voltage. Once a thyristor is triggered by the application of a predetermined current to its gate, the forward blocking impedance is lowered, thereby permitting the flow of electrical energy through the thyristor in the manner of a diode. Once conduction has been enabled, the thyristor cannot be turned off until the current flowing therethrough is reduced to near zero (i.e., makes a zero crossing).
The amount of power transferred to the motor is controlled by varying the duration of the conduction of the various thyristors. This is done by controlling the firing angle of each thyristor, that is, the point in the AC waveform at which the thyristor is initiated into conduction. The process of switching from thyristor to thyristor is known as commutation.
Generally, in a polyphase system, all of the line current is carried by a particular phase or thyristor at a given point in time. However, since the commutation of current from one thyristor cell to the next takes both time and voltage, during commutation, two consecutive cells become shorted for the time it takes to commute the line current from the outgoing cell to the oncoming cell. The duration of this short is related to both the commutation inductance (the inductance of the source supplying the bridge) and the amplitude of the current being commutated. As a result, the typically sinusoidal AC line to line voltage becomes corrupted by periods of zero voltage caused by the cell shorting. These disturbance periods are commonly referred to as voltage notches. Since six thyristors are fired during each cycle, six voltage notches appear in each of the line to line voltage signals every cycle.
In order to control the firing of the various thyristors, conventional systems incorporate a firing controller, either analog or digital, to control the firing angle of each thyristor. The line to line voltages described above are generally used by the controller in conjunction with a phase lock loop (PLL) to synchronize the phases of each of the received line to line voltages and to thereby accurately determine the proper thyristor firing angles. The above-described voltage notches in each of the line to line voltage signals introduce deleterious effects to the accuracy and stability of the PLL and the firing controller.
U.S. Pat. No. 4,399395 issued to Paul M. Espelage recognizes problems caused by voltage notches and discloses a method for compensating therefor wherein a synchronizing pulse train is generated from a composite waveform developed by summing at least one line to line voltage signals containing the voltage notches and a signal corresponding to at least one delta current derived from the difference of two phase currents and multiplied by a factor representative of the commutation inductance. Unfortunately, this method fails to efficiently and accurately compensate for such effects with a minimum of complexity and increased cost.
Accordingly, there is a need in the art of power control systems for a system and method for accurately compensating for the effects of voltage notches without increasing the cost or complexity of the system.